Die neuen 3D-Entwurfs-, Berechnungs- und Fertigungsverfahren in Kombination mit dem Werkstoff ultrahochfester Beton (UHPC) bieten das Potenzial, den Beton-Leichtbau zu revolutionieren [1]. Die Herausforderung bei der Herstellung von geometrisch komplexen und hochpräzisen UHPC-Bauteilen liegt dabei im Schalungsbau. Da bisher keine verfügbaren abfallfreien und somit nachhaltigen alternativen Schalungsmaterialien bzw. -systeme identifiziert werden konnten, wurde der Forschungsansatz entwickelt, frei geformte Schalungen für Betonbauteile unter Verwendung von CNC-gefrästen recycelbaren Industriewachsen zu verwenden. Die Erforschung dieses Ansatzes hin zu einer anwendbaren Non-Waste-Schalungstechnologie wurde in einem gemeinsamen Forschungsprojekt des Instituts für Werkzeugmaschinen und Fertigungstechnik (IWF) und des Instituts für Tragwerksentwurf (ITE) der TU Braunschweig durchgeführt. Im Folgenden werden die wesentlichen Inhalte des Vorhabens, ausgehend von der Auswahl geeigneter Wachse, über die Untersuchung der Zerspanbarkeit bis hin zur Betonierung und anschließenden Analyse der Schalungen und Abgüsse, vorgestellt und diskutiert. Grundlegende Erkenntnisse wurden u. a. bereits 2016 in [2]–[5] veröffentlicht. Diese werden hier teilweise wiedergegeben und zudem mit zusätzlichen Informationen ergänzt. Die wesentlichen Erkenntnisse aus dem Forschungsvorhaben werden zusammengefasst. Ausführliche Informationen zur Entwicklung der Non-Waste-Wachsschalungstechnologie finden sich in der 2019 veröffentlichten Dissertation von Jeldrik Mainka [6]. / The new 3D design, calculation and manufacturing methods in combination with ultra-high strength concrete (UHPC) off er the potential to revolutionise lightweight concrete construction [1]. The challenge in the production of geometrically complex and high-precision UHPC components lies in formwork construction. As no available waste-free and thus sustainable alternative formwork materials or systems have been identified so far, the research approach was developed to use freely shaped formwork for concrete components using CNC-milled recyclable industrial waxes. The research of this approach towards an applicable non-waste formwork technology was carried out in a joint research project of the Institute for Machine Tools and Production Engineering (IWF) and the Institute of Structural Design (ITE) of the Technical University of Braunschweig. In the following, the main contents of the project, starting with the selection of suitable waxes, the investigation of machinability up to the concreting and subsequent analysis of the formwork and castings are presented and discussed. Basic findings have already been published in 2016 in [2]–[5]. These are partly reproduced here and supplemented with additional information. The main findings of the research project are summarised. Detailed information on the development of non-waste wax formwork technology can be found in the dissertation by Jeldrik Mainka [6], published in 2019.

There was developed a novel technological and constructive approach for the low-cost production of curved freeform formworks, which allow the production of single and double-curved fibre reinforced concrete. The scheduled approach was based on a flexible, asymmetrical multi-layered formwork system, which consists of glass-fibre reinforced plastic (GFRP). By using of the unusual anisotropic structural behavior, these GFRP formwork elements permitted a specific adjustment of defined curvature. The system design of the developed GFRP formwork was examined exhaustively. There were designed, numerically computed and produced prototypical curved freeform surfaces with different curvature radii. The fibre reinforced concrete had a compressive strength of 101.4 MPa and a 3-point bending tensile strength of 17.41 MPa. Beyond that, it was ensured that the TRC had a high durability, which has been shown by the capillary suction of de-icing solution and freeze thaw test with a total amount of scaled material of 874 g/m² and a relative dynamic E-Modulus of 100% after 28 freeze-thaw cycles.

O presente estudo refere-se a uma das deteriorações mais freqüentes nas estruturas de concreto armado: a ação da carbonatação. Para essa verificação construiu-se uma câmara de carbonatação acelerada, que foi calibrada, com a finalidade de estudar o processo de carbonatação em corpos-de-prova executados em concreto e argamassa com cimentos CP V ARI Plus e CP V ARI RS com e sem adição de sílica ativa, curados em câmara úmida por sete dias e posteriormente expostos à atmosfera agressiva de gás carbônico por 7, 14, 28, 63 e 91 dias. Também foram executados corpos-de-prova semelhantes (controle), os quais foram ensaiados à compressão axial e à compressão diametral para determinação da resistência à compressão, tração e medida da profundidade de carbonatação. A partir destes resultados é ajustado um modelo teórico experimental para previsão da profundidade de carbonatação em função do tempo. Observa-se que para todas as composições estudadas a profundidade de carbonatação é pequena. Analisa-se também, a influência da carbonatação no ganho da resistência mecânica das argamassas e dos concretos, e o efeito da adição de sílica ativa e do tipo de cimento no fenômeno de carbonatação. Finalizando, é apresentada uma justificativa dos resultados com base no banco de dados existente no LMABC-SET-EESC-USP. / The present study refers to one of the most frequent deterioration in reinforced concrete structure: the action of carbonation. For this, an accelerated carbonation chamber was built and gauged in order to study the carbonation process in concrete and mortar specimens with CP V ARI Plus and CP V ARI RS cements, with and without silica fume addition. The specimens were cured in a humidity chamber for seven days and exposed to aggressive atmosphere of carbonic gas for 7, 14, 28, 63 and 91 days. Similar specimens of control were also manufactured and left in humidity chamber during the same periods. These specimens were tested an axial compression and splitting tensile strength to determine the compression and tensile strength and the carbonation depth. From all the obtained results an experimental theoretical model was forecasted to determine the depth carbonation in function of time. It is observed that all the depths carbonation measured is small. The carbonation influence on mechanical resistance gain of the mortar and concrete, as well as the effect addition of both of silica fume and cement type on the phenomenon of carbonation is also analyzed. Finally, it is presented a justification of results based on the existent database at LMABC-SET-EESC-USP.

[pt] Projetos estruturais que empregam concretos com resistências mais elevadas, ou seja, maiores que 40 MPa, nomeados Concretos de Alto Desempenho, são cada vez mais freqüentemente utilizados. Sabe-se, entretanto, que as propriedades destes concretos são diferenciadas daquelas dos concretos considerados convencionais, necessitando-se estudos que visem, principalmente, mensurar a realidade dentro dos materiais disponíveis em nosso país. Este trabalho experimental estuda as principais propriedades deste material: resistência à compressão, resistência à tração por compressão diametral, Módulo de Elasticidade e permeabilidade, através da determinação do índice de vazios. Após um estudo sobre métodos de dosagem mais utilizados e empregando-se materiais disponíveis no Rio de Janeiro, foram moldados um total de 230 corpos de prova para resistências características à compressão de 40MPa, 60 MPa, 80 MPa e 100 MPa. Foi realizado um estudo sobre compatibilidade entre superplastificante e aglomerante. Com os dados obtidos experimentalmente, foram relacionados resistências à tração por compressão diametral, módulo de elasticidade e porosidade com resistência à compressão axial. Estes resultados foram comparados com as recomendações das principais normas internacionais e com as propostas de alguns pesquisadores que foram encontradas na literatura. / [en] High Performance Concrete, i.e., those whose compressive strength is higher than 40 Mpa, have been used in a wide range of Civil Engineering applications. Notwithstanding that, it is known that their mechanical and physical properties are different from those of their conventional counterparts. In this dissertation, it is studied mixture proportion methods with good acceptance rate, preparing 230 specimens with materials available in Brazil s state of Rio de Janeiro and that have compressive strength of 40 MPa, 60MPa, 80 MPa and 100 MPa. For each prepared specimen, it is studied its mechanical properties, compressive strength, splitting tensile strength, modulus of elasticity and porosity for the determination of the emptiness ratio. It is also studied the compatibility between superplasticizer and cement materials. It is also performed a comparison between the experimental data and the expressions available in the literature, as well as the relation between the compressive strength, splitting tensile strength, modulus of elasticity and the porosity.

Treten im Beton mehraxiale Spannungszustände auf, führen diese gegenüber einer einaxialen Beanspruchung zu einer signifikanten Änderung des Materialverhaltens. Neben einer festigkeitssteigernden bzw. -abmindernden Wirkung ergeben sich ebenfalls große Unterschiede im Spannungs-Dehnungs-Verhalten. Zur effizienten Konzipierung von Betonstrukturen unter komplexen Beanspruchungszuständen ist daher die Kenntnis des veränderten Materialverhaltens notwendig. Zur experimentellen Bestimmung des Spannungs-Dehnungs-Verhaltens eines Ultrahochleistungsbetons mit einer einaxialen Druckfestigkeit von über 170 N/mm² wurden mehraxiale Belastungsversuche an würfelförmigen Probekörpern durchgeführt. Die Untersuchung umfasste insgesamt 35 zwei- und dreiaxiale Spannungsverhältnisse unter proportionaler Laststeigerung mit vorrangiger Betrachtung von Zug-Druck-Druck-Beanspruchungen. Für die Einleitung der Zugbeanspruchungen in die Prüfkörper wurde eine neue Methode entwickelt, bei der mittels einbetonierter Schrauben die Belastung auf den Beton übertragen wird. Die Bestimmung des Verformungsverhaltens erfolgte im Inneren der Probekörper mit sechs tetraederförmig angeordneten Faser-Bragg-Gittern. Die somit direkt gemessenen Dehnungen ermöglichen die nachträgliche Berechnung der Komponenten des Dehnungstensors des Bezugskoordinatensystems. Für den untersuchten Ultrahochleitsungsbeton fallen die auf die einaxiale Druckfestigkeit bezogenen mehraxialen Festigkeitswerte mit zunehmendem hydrostatischen Druckspannungsanteil der Beanspruchung geringer aus als bei Normalbetonen. Weiterhin weist das Verformungsverhalten eine größere Sprödigkeit gegenüber Normalbetonen auf, so dass auch unter dreiaxialen Druckspannungszuständen die Probekörper schlagartig versagen. Aus den gemessenen Spannungs-Dehnungs-Linien werden neben den maximalen Festigkeiten die Festigkeitswerte an der Elastizitätsgrenze, der Affinitätsgrenze sowie beim Volumenminimum der Probekörper bestimmt. Zur Approximation dieser charakteristischen Werte wurde eine Grenzflächenbeschreibung entwickelt und an den Versuchsergebnissen kalibriert. Des Weiteren erfolgte die Zusammenstellung einer Datenbank mit in der Literatur verfügbaren mehraxialen maximalen Festigkeitswerten von Betonen mit einaxialen Druckfestigkeiten von 10 N/mm² bis 180 N/mm² und die Kalibrierung des entwickelten Modells zur Grenzflächenbeschreibung in Abhängigkeit der einaxialen Druckfestigkeit. Die bei der Kalibrierung der Grenzfläche für einzelne Betonfestigkeitsklassen bestimmten Freiwerte hängen dabei stark von den vorliegenden Versuchsdaten und speziell vom Wertebereich der hydrostatischen Spannungsanteile der maximalen Beanspruchungen ab. Die Approximation des Spannungs-Dehnungs-Verhaltens der mehraxial beanspruchten Probekörper erfolgt mittels eines schädigungsbasierten Materialgesetzes. Hierbei wird für den anfänglich isotropen Beton zum einen eine lastinduzierte isotrope Schädigung und zum anderen eine lastinduzierte orthotrope Schädigung angenommen, die von den auftretenden Hauptdehnungen abhängig ist. Mit dem entwickelten Materialgesetz werden sehr gute Übereinstimmungen mit den gemessenen Spannungs-Dehnungs-Linien erreicht, so dass sich ebenfalls eine gute Vorhersage der maximalen Festigkeitswerte ergibt. / Concrete under multiaxial stress states shows significant changes of the material behaviour compared to uniaxial loading. Besides strength increasing and decreasing effects, also great differences in the stress-strain behaviour occur. In order to design concrete structures efficiently concerning complex stress states, the knowledge about the modified material behaviour is necessary. To determine experimentally the stress-strain behaviour of an ultra high performance concrete with a uniaxial compressive strength of about 170 N/mm², multiaxial loading tests on cubic-shaped specimens were carried out. Altogether, the investigation contained 35 biaxial and triaxial stress ratios under proportionally increasing load with primarily tension-compression-compression loadings. Applying the tensile load on the specimen, a new method was developed, which uses screws embedded in the concrete to transfer the loading. The deformations were measured by using six tetrahedron-shaped arranged Fibre Bragg Gratings inside the concrete specimen. Subsequently, with the directly measured strains the components of the strain tensor of the reference coordinate system could be determined. For the investigated ultra high performance concrete the increase of the multiaxial strength, referring to the uniaxial compressive strength, decreases compared to normal strength concrete with the increasing hydrostatic stress component of the load. Moreover, the deformation behaviour shows an increased brittleness compared to normal strength concrete, so that even under triaxial compressive stress states the specimens fail abruptly. Besides the ultimate strength, from the measured stress-strain curves the strength at the proportional limit, at the limit of affinity as well as at the minimum volume of the specimen is determined. To approximate these characteristic values, a description of a hypersurface is developed and calibrated with the test results. Furthermore, a database with multiaxial ultimate strength values of concretes with uniaxial compressive strengths between 10 N/mm² to 180 N/mm² available from literature was compiled and a calibration of the developed hypersurface model depending on the uniaxial compressive strength was carried out. Thereby, the obtained values of arbitrary parameters of individual concrete strength classes depend severely on the available test results, especially on the range of values of the hydrostatic stress component of the ultimate strength. The approximation of the stress-strain behaviour of the multiaxial loaded specimens is carried out by means of a damage-based material law. For this purpose, concerning the initially isotropic concrete, a load-induced isotropic and orthotropic damage depending on the principle strains is assumed. With the developed material law, very good accordance with the measured stress-strain curves could be achieved, so that also results in a good approximation of the ultimate concrete strength.

The aim of this diploma thesis is to focus on the resistance of concrete exposed to high temperatures especially with focus of resistance against explosive spalling as well as clarifying the mode of action of various types of polypropylene fibres. The theoretical part is an introduction to the issues of explosive spalling, its mechanisms and majority influencing factors. Further description of the processes taking place in the structure of concrete under extreme thermal load, distribution and size of pores in concrete, thermal load, temperature-time curves and their applications, methods of elimination of negative behaviour of concrete exposed to thermal loading (passive and active methods), mode of action of polypropylene fibres and more is also included in the theoretical part. The main aim of experimental part is to verify the function of polypropylene fibers of various Melt Flow Indexes (MFI) and dosage. Primarily, the test samples with content of the PP-fibers are compared to the reference sample without fibers. Secondarily, the samples with standard PP-fibers (with MFI 25) with dosage 2.0 kg/m3 are compared to samples with modified PP-fiber (with MFI 2500) of dosage 0.9 kg/m3. Photogrammetric images were used for evaluation and comparison of spalled surfaces and its depth. Determination of the softening temperature and melting point of the modified and standard PP-fibers was made by using a high temperature microscope video. The second part of the experimental work was to define concrete permeability at different temperatures and pressures. Permeability was measured at temperatures of 20°C, 90°C, 150°C, 175°C, 200°C, 225°C and 250°C and at pressure of 0.2, 0.4 and 0.6 MPa.

Bei den Forschungsaktivitäten wird von aktuellen Fragestellungen der Baustofftechnologie ausgegangen, wobei die Lösung akuter Probleme der Baupraxis und die Schaffung von soliden theoretischen Grundlagen in gleichem Maße angestrebt werden. Die Forschung wird hierbei vor allem durch interdisziplinäres Arbeiten geprägt. Zu den aktuellen Forschungsschwerpunkten zählen insbesondere: Entwicklung neuer zementbasierter Verbundwerkstoffe sowie von Verfahren zu deren Herstellung mit besonderem Akzent auf Faserbetone (Hochduktiler Beton mit Kurzfasern, Beton mit textiler Bewehrung, Ultrahochfester Beton mit innerer Nachbehandlung, Selbstverdichtender Leichtbeton, Beton mit sehr hohem Verschleißwiderstand) Untersuchung der Kurz- und Langzeiteigenschaften von neuen und bestehenden Baustoffen auf mineralischer Basis (Beton, Mörtel, Mauerwerk); Erforschung der für das Materialverhalten maßgebenden Mechanismen sowie der Mittel zu deren gezielten Beeinflussung (Festigkeits-, Verformungs- und Bruchverhalten unter monotoner, zyklischer und stoßartiger Beanspruchung sowie Schwinden und Kriechen von Beton; Transport von korrosiven Medien unter Berücksichtigung der Rissbildung, Schädigungsmechanismen und Dauerhaftigkeit) Modellierung des Baustoffverhaltens; Ableitung von stoffgesetzlichen Beziehungen; numerische Simulation des Materialverhaltens in unterschiedlichen Stadien seines "Lebens" (Herstellung, Verarbeitung, Erhärtung, mechanische Beanspruchung, Exposition von korrosiven Medien etc.)

O presente estudo refere-se a uma das deteriorações mais freqüentes nas estruturas de concreto armado: a ação da carbonatação. Para essa verificação construiu-se uma câmara de carbonatação acelerada, que foi calibrada, com a finalidade de estudar o processo de carbonatação em corpos-de-prova executados em concreto e argamassa com cimentos CP V ARI Plus e CP V ARI RS com e sem adição de sílica ativa, curados em câmara úmida por sete dias e posteriormente expostos à atmosfera agressiva de gás carbônico por 7, 14, 28, 63 e 91 dias. Também foram executados corpos-de-prova semelhantes (controle), os quais foram ensaiados à compressão axial e à compressão diametral para determinação da resistência à compressão, tração e medida da profundidade de carbonatação. A partir destes resultados é ajustado um modelo teórico experimental para previsão da profundidade de carbonatação em função do tempo. Observa-se que para todas as composições estudadas a profundidade de carbonatação é pequena. Analisa-se também, a influência da carbonatação no ganho da resistência mecânica das argamassas e dos concretos, e o efeito da adição de sílica ativa e do tipo de cimento no fenômeno de carbonatação. Finalizando, é apresentada uma justificativa dos resultados com base no banco de dados existente no LMABC-SET-EESC-USP. / The present study refers to one of the most frequent deterioration in reinforced concrete structure: the action of carbonation. For this, an accelerated carbonation chamber was built and gauged in order to study the carbonation process in concrete and mortar specimens with CP V ARI Plus and CP V ARI RS cements, with and without silica fume addition. The specimens were cured in a humidity chamber for seven days and exposed to aggressive atmosphere of carbonic gas for 7, 14, 28, 63 and 91 days. Similar specimens of control were also manufactured and left in humidity chamber during the same periods. These specimens were tested an axial compression and splitting tensile strength to determine the compression and tensile strength and the carbonation depth. From all the obtained results an experimental theoretical model was forecasted to determine the depth carbonation in function of time. It is observed that all the depths carbonation measured is small. The carbonation influence on mechanical resistance gain of the mortar and concrete, as well as the effect addition of both of silica fume and cement type on the phenomenon of carbonation is also analyzed. Finally, it is presented a justification of results based on the existent database at LMABC-SET-EESC-USP.

Roofs are the most vulnerable part of the building envelope that often get damaged when subjected to hurricane winds. Damage to the roofs has a devastating impact on the entire structure, including interior losses and service interruptions. This study aimed at the development of a novel light-weight composite flat roof system for industrial, commercial and multi-story residential buildings to withstand Category 5 hurricane wind effects based on the Florida Building Code requirements for hurricane-prone regions, the strictest wind design code in the United States. The proposed roof system is designed as a combination of two advanced materials: ultra-high performance concrete (UHPC), reinforced with high strength steel (HSS). The novel combination of these two materials in a specially designed cross section led to a lightweight low-profile ultra-thin-walled composite roof deck, with only 17 pounds per square foot self-weight, 4-inch overall depth and only ¾-inch thick flanges and webs, with no shear reinforcement or stirrup. Two groups of specimens, single-cell and multi-cell, were fabricated and tested in four-point flexure to determine the ultimate bending capacity and ductility of the system. Each group of specimens included two short-span (9 ft.) samples (due to the laboratory constraints) -- one specimen subjected to positive bending and the other one subjected to negative bending, representing the critical loading conditions including the effects of wind pressures. All specimens exhibited pure flexural failure in a ductile behavior and with no sign of shear failure. Finite element models of laboratory specimens were also developed and calibrated based on experimental data in order to project the performance of the system for larger and more realistic spans. The experimental work and the finite element analyses showed that the proposed roof system with its given section has adequate flexural and shear strength, and also meets serviceability requirements for a 20-foot long span. Moreover, connections for the roof system were proposed, including panel-to-panel connections and roof-to-wall connections. In addition to safety, the other advantages of the proposed roof system in comparison to the equivalent reinforced concrete roofs include a three-fold reduction in self-weight, a three-fold reduction in overall profile height, and a five-fold reduction of steel reinforcement. Together, these advantages may lead to an increased span length beyond what is typically feasible for the conventional reinforced concrete slabs. All these features translate the proposed deck to a sustainable roof system.

L’étude du comportement des bétons à hautes températures est d’une grande importance pour déterminer la stabilité et la résistance des structures en béton en situation d’incendie. Des incendies spectaculaires, ces dernières années, ont montré que le béton pouvait être très fortement sollicité. Ainsi il apparaît qu’assurer un très bon comportement du béton en température permet de sauver des vies et de diminuer les frais d'immobilisation et de réparation des structures endommagées. Le travail présenté vise, par une étude expérimentale, à apporter une meilleure compréhension des phénomènes mis en jeu lors de l’exposition aux hautes températures des bétons à hautes performances (BHP). Il s’intéresse, plus particulièrement, aux transferts des fluides dans les bétons (perméabilité) et à l’étude de l’instabilité thermique (phénomène d’éclatement) des bétons à hautes performances. Le travail de thèse a été mené sur des bétons à hautes performances (80 MPa) avec 5% de fumée de silice (matériau à faible perméabilité pour améliorer la durabilité), et différents pourcentages de fibres de polypropylène. L’utilisation des fibres de polypropylène (PP) est considérée comme une solution technique efficace pour améliorer la tenue au feu des bétons à hautes performances, notamment grâce aux phénomènes physiques apparaissant aux températures inférieures à 200°C. Notre travail concerne, dans un premier temps, la caractérisation de l’évolution des propriétés physiques (porosité, perte de masse), thermiques (conductivité et diffusivité) et mécaniques (résistance à la compression et en traction) ainsi que les isothermes de sorption. La deuxième partie de ce travail concerne l’étude de l’évolution de la perméabilité en fonction de la température (jusqu’à 200°C), principale propriété caractérisant les transferts hydriques au sein des bétons. Les paramètres étudiés sont : le type de béton (avec ou sans fibres de PP), deux types de fibre de géométries différentes, la nature du fluide (air ou vapeur d’eau) et la méthode de mesure (en résiduel et à chaud). Les résultats expérimentaux montrent que les fibres génèrent une perméabilité plus importante au-delà de 150°C et que les valeurs obtenues en résiduel ou à chaud peuvent être liés aux modes opératoires. Enfin, dans la dernière partie une investigation sur le phénomène d’éclatement explosif en lien avec la composition du matériau est menée. Une nouvelle méthode d’investigation expérimentale de la sensibilité des formulations à l’instabilité thermique est proposée avec une étude sur des sphères portées en température. Cette méthode devra être comparée aux méthodes traditionnelles d’investigation. L’analyse des résultats des essais permet de discuter sur les causes de l’instabilité thermique et de caractériser les propriétés du béton en température, données indispensables à la modélisation. / The study of the behaviour of concrete at high temperatures is of great importance to determine the stability and strength of concrete structures in fire conditions. Spectacular fires, these last years, showed that the concrete could be very strongly solicited. Thus it appears that to ensure a very good behaviour of the concrete at temperature it is saved people and reduced the expenses of immobilisation and repair of the damaged structures. The aims of the present work, by the experimental study, are to provide a better comprehension of the involved phenomena during the exposure to the high temperatures of the high performance concretes (HPC). It focuses, in particular, on the fluids transfers (permeability) and the study of the thermal instability (spalling) of high performance concrete. The thesis work was conducted on high-performance concrete (80 MPa) with 5% silica fume (low permeability material to improve durability), and different amount of polypropylene fibres. The use of polypropylene fibres (PP) is regarded as an effective technical solution to improve the behaviour of the HPC at fire conditions, in particular thanks to the physical phenomena appearing at the temperatures lower than 200°C. Our work is concerned, initially, on the characterization of the evolution of physical properties (porosity, mass loss), thermal properties (conductivity and diffusivity) and mechanical properties (compressive strength and tensile) and the sorption isotherms. The second part concerns the study of the evolution of the permeability, as a main property characterising the hydrous transfers inside the concrete exposed to temperature. An experimental study is conducted concerning the evolution of the permeability of concrete as a function of temperature (up to 200 ° C). The studied parameters are: the type of concrete (with and without fibres), two types and geometry of fibres, the nature of the fluid (air and vapour) and the measurement methods (residual and under temperature). The experimental results shows that the existing fibres generate a most important permeability beyond 150°C and that the obtained results, by residual or under temperature measurements, may be related to the operating procedures. Finally, in the last part an investigation on the spalling phenomenon related to the material compositions is conducted. A new experimental method of investigation of the sensitivity of the formulations to thermal instability is proposed: the study of spherical specimens exposed in high temperatures. This method will be compared to the traditional methods of investigation. The analysis of the test results makes to discuss the thermal instability causes and to characterize the concrete properties at high temperatures, important data to modelling.